Exposure Mesh Back Face Contribution

Rays falling on the back face of a mesh appear to contribute to some nodes on the edge only. Is this a bug? Is there any way around this so that only one face contributes? I am pretty new to grasshopper so not sure.

The picture below shows two rays falling on either side of a mesh, with vectors coming out of each vertex scaled by calculated exposure. The vectors on the left hand edge are smaller than the others because they are getting a contribution from the ray falling on the back of the mesh.ExposureTest.gh

Any help with this will be much appreciated.

Rob Murphy

London